1. Field of the Invention
The present invention relates to a multi-beam scanning apparatus.
2. Description of Related Art
Conventional multi-beam scanning apparatuses include a plurality of light sources which are capable of being independently driven to modulate each beam according to an image signal and a plurality of beams emitted from the light sources scan a plurality of lines simultaneously on a scanning surface. The beams emitted from the light sources are deflected by a common deflector at equiangular velocity, and are then condensed by a common scanning image forming lens so as to form optical spots on the scanning surface. These optical spots are separated from each other in a sub-scanning direction. As a result, the plurality of lines are simultaneously scanned by the beams on the scanning surface at a substantially constant velocity.
In such multi-beam optical scanning apparatuses, a semiconductor laser or an LED is generally used as the light sources. The wavelength of a beam radiated from a semiconductor laser or an LED generally differs from that from other semiconductor lasers or LEDs. That is, when multiple light sources are used, there is generally some difference in wavelength of light radiated from the multiple light sources. Therefore, when a plurality of semiconductor lasers or LEDs are used as the light sources in a multi-beam scanning apparatus, the radiation wavelength of each light source is generally not the same as that of the other light sources. This difference in wavelength of the different light sources is caused by manufacturing tolerances. When the radiation wavelength of each light source is different from that of the other light sources, the magnification with a scanning image forming lens differs for each light source due to the color aberration of the lens. For the reasons described below, this difference in wavelength was not a serious problem with previous multi-beam scanning apparatuses because the image forming resolution was not high enough that the difference in wavelength would produce problem.
FIG. 1 is a schematic drawing illustrating an exemplary construction of a multi-beam scanning apparatus having a plurality of light sources. In FIG. 1, reference numerals 11, 12 denote a semiconductor laser which defines a light source in this apparatus. The semiconductor lasers 11, 12 are independently driven to modulate a light beam according to an image signal, respectively. The beams emitted from the semiconductor lasers 11, 12 are converted by coupling lenses 13, 14 into beam shapes suitable for a subsequent optical system. The beams passed through the coupling lenses 13, 14 are condensed in the sub-scanning direction by a cylindrical lens 15 so as to be formed as linear images elongated in the main scanning direction.
A rotating polygon mirror which functions as a deflector 16 is arranged such that a reflective deflecting plane thereof for reflecting each beam is located in the vicinity of a location where the linear images are formed. Each beam reflected by the reflective deflecting plane of the deflector 16 is deflected at equiangular velocity as the deflector 16 rotates at a constant velocity. The deflected beams then pass through a scanning image forming lens 17 and are condensed by the effect of the scanning image forming lens 17 forming optical spots on a scanning surface 18. The optical spots on the scanning surface 18 are separated from each other in the sub-scanning direction. The scanning surface 18 is typically a photoconductive element and a latent image is written on the scanning surface 18 as a result of being scanned by the optical spots.
The beams deflected at equiangular velocity by the deflector 16 are detected by a synchronous detector including a lens 19 and an optical sensor 20 before being deflected toward a writing area of the scanning surface 18. The beams are first condensed by the lens 19 and are then detected by the optical sensor 20. The two beams are separated from each other in the main scanning direction also and are individually detected by the synchronous light detect device. The start timing of writing information on the scanning surface 18 by each beam is synchronized with each other a ccording to the detection result of the synchronous light detector.
Optical scanning with each optical spot starts from a writing start position BG on the scanning surface 18 and information of one scanning line is written in a predetermined period of time. The length in which information of one line is written is referred to as "writing width". The writing width is generally determined as a design value for an apparatus.
The radiation wavelength of a light source is determined by the specific characteristics of a semiconductor laser used as the light source. The value of the radiation wavelength used as a design value is referred to as the desired criterion wavelength and is represented by .lambda.. An example of the desired criterion wavelength is, for example, 780 nm, which is a typical wavelength of a semiconductor laser.
The lens effect of the scanning image forming lens 17 depends on the wavelength of a passing light. Therefore, the writing width depends on the radiation wavelength of the light source, and the writing width as a design value is determined by the desired criterion wavelength .lambda.. Therefore, the writing width is herein represented by the criterion writing width L(.lambda.).
The radiation wavelength of each light source varies as described above. When the radiation wavelength of a light source is deviated from the desired criterion wavelength .lambda. to (.lambda.+/-.DELTA..lambda.), .DELTA..lambda. representing a small change in the wavelength, the actual writing width L(.lambda.+/-.DELTA..lambda.) differs from the above criterion writing width L(.lambda.).
As described above, each beam is detected by the synchronous light detector before being deflected toward the writing area of the scanning surface 18 and thereby, the writing start position of each beam is synchronized with each other. Because the beam detect position which is determined by the synchronous light detector and the writing start position BG are relatively close to each other, the writing start position BG of each beam is substantially the same for each beam and is not deviated from each other. However, at the writing end side, because of the effect of the magnification difference due to the color aberration or chromatic aberration of the scanning image forming lens 17, the writing end position FN differs between the light sources to a degree causing a problem described next.
In the apparatus of FIG. 1, in which two beams from two light sources scan the scanning surface 18, when the optical spots scan adjacent lines, respectively, as illustrated in FIG. 2, the writing end position FN1 of one of the two optical spots and the writing end position FN2 of the other optical spot are deviated from each other. Therefore, when a vertical line, i.e., a straight line in the sub-scanning direction, is written at the writing end position, the line is not written straight but instead has a non-uniform, wavy pattern as illustrated in FIG. 2.
For example, when two semiconductor lasers having the desired criterion wavelength of 780 nm are used in the scanning apparatus of FIG. 1, because the deviation of the radiation wavelength of the semiconductor lasers .DELTA..lambda. is generally +/-20 nm, it is conceivable that the radiation wavelength of one of the semiconductor lasers is 800 nm and that of the other semiconductor laser is 760 nm. When the criterion writing width L for the semiconductor laser having the wavelength of 780 nm is assumed to be 216 mm, the writing width L may deviate by about 70 .mu.m between the semiconductor lasers having the radiation wavelengths of 800 nm and 760 nm, respectively.
When the writing resolution is 400 dpi (dot-per-inch), the size of one dot as a unit of writing information with optical scanning is about 63.5 .mu.m in diameter. In this case, because the above deviation of 70 .mu.m from dot locations in the writing end position of the two beams is about the same, the above-described wave of a straight line is not recognizable.
Recently, the demand for improving the writing resolution has increased and the higher writing resolution, such as 600 dpi and 1200 dpi, is now being realized. When a vertical straight line is written in the sub-scanning direction with the writing resolution of 600 dpi, the size of one dot being 40 .mu.m in diameter, the above deviation amount of 70 .mu.m as the maximum amplitude of the wave of a straight line is about twice as large as the size of the writing dot and therefore, the wave in a desired straight line starts to become recognizable.
The inventors of the present invention discovered that the wave in the desired straight line starts to be recognizable when the maximum amplitude of the wave of a straight line is more than about twice as large as the size of one writing dot.
The magnification difference in a scanning image forming lens can be caused by other factors besides the color aberration of the lens. As described above, the plurality of beams from the plurality of light sources deflected by the deflector are separated from each other in the sub-scanning direction. Accordingly, the positions in the scanning forming lens where the beams pass are different. This difference also causes the magnification difference between the beams, which is herein called the magnification difference due to the difference in the passing position in the scanning image forming lens.
The above magnification difference between beams from a plurality of light sources caused by chromatic or color aberration of a scanning image forming lens can be corrected by making the lens colorless, but this increases the cost of the scanning image forming lens. Further, when the scanning image forming lens includes a plastic lens, correction of the color aberration is difficult because there are only a few types of plastic materials suitable for use in forming the optical lens.
In a specific conventional prior art device described in JP 9-76562, the wavelengths of two optical elements used as light sources are required to be within 12 nm of each other so as to minimize vertical line waving as seen in FIG. 2 of the present application so that the line waving is not so visible. In order to achieve this wavelength variation of only 12 nm is to have a very precise arrangement of the light sources and apparatus elements. More specifically, the wavelength for each specific light source must be accurately measured and then compared to the wavelength of other light sources to find a suitable match that makes a difference between the wavelengths of the selected two light sources less than 12 nm. It is extremely difficult and time-consuming to find two light sources that have a wavelength difference of less than the required difference of 12 nm. Thus, this solution to the problem of the waviness in the vertical-line is very difficult, time consuming and makes the assembly process very expensive.